Fifty patients with mild or moderate essential hypertension were randomized (double-blindly) to treatment with either captopril (n = 26) or atenolol (n = 24). Their mean supine diastolic blood pressure after placebo was 100-125 mmHg. The study included an initial dose finding phase (12 weeks) during which the dosages of captopril and atenolol were increased stepwise every second week in order to obtain normotension (supine diastolic blood pressure less than 95 mmHg). Hydrochlorothiazide was added when necessary. During the second phase of the study the patients were followed on active treatment for 2 years. After the initial 12 weeks of active treatment, recumbent and standing blood pressures had fallen significantly both in the captopril group (by 31/20 and 33/19 mmHg, p less than 0.001) and in the atenolol group (by 24/18 and 30/20 mmHg, p less than 0.01 (systolic), p less than 0.001 (diastolic)). The antihypertensive effect was maintained in both groups during long-term treatment. The antihypertensive effect of both agents was potentiated to the same extent by addition of hydrochlorothiazide. Side-effects were few and mild. It can be concluded that both captopril and atenolol are safe and effective antihypertensive drugs.